Added "rest" argument to wxString::Before{First,Last}().

Added "rest" argument to wxString::Before{First,Last}().

This allows to search the string just once, in BeforeXXX(), when both the
parts of the string before and after some character are needed instead of
having to do it twice in both BeforeXXX() and AfterXXX().

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65862 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775